Nitrogen doping was recently shown to extend the absorptivity of TiO 2 photocatalysts into the visible. We find that N-doped TiO 2 materials fail, however, to catalyze the oxidation of HCOOinto CO 2 •-, or of NH 3 -OH + into NO 3 -, under visible illumination. By N-doping anatase at ambient or high temperature according to the literature we obtained yellow powders A and H, respectively, that absorb up to ∼520 nm. Aqueous H suspensions (pH ∼ 6, 1 atm O 2 ) photocatalyze the oxidation of HCOOinto CO 2 •radicals at λ ∼ 330 nm, but the quantum yield of CO 2 •formation at λ > 400 nm remains below ∼2 × 10 -5 and is probably zero. A is similarly inert toward HCOOin the visible region and, moreover, unstable in the UV range. Thus, the holes generated on N-doped TiO 2 by visible photons are unable to oxidize HCOOeither by direct means or via intermediate species produced in the oxidation of water or the catalyst. Reports of the bleaching of methylene blue (MB) on N-doped TiO 2 , which may proceed by direct oxidative or reductive photocatalytic pathways and also by indirect photocatalysis (i.e., induced by light absorbed by MB rather than by the catalyst) even under aerobic conditions are, therefore, rather uninformative about the title issue.

Direct experimental evidence of the higher concentration of hydroxyl radicals generated on fluorinated titanium dioxide (F-TiO2) under irradiation was obtained by spin-trapping EPR measurements. The faster photoinduced bleaching of the azo dye Acid Red 1 (AR1) observed in the presence of F-TiO2 was explained by the high affinity of the azo double bond towards *OH radicals. Moreover, the pronounced decrease of the AR1 bleaching rate by addition of 2-propanol, as hydroxyl radicals scavenger, on F-TiO2 and not on naked TiO2 demonstrated that on fluorinated titania AR1 is mainly degraded via *OH radical attack.

The degradation of 2-chlorophenol and of the two azo dyes acid orange 8 and acid red 1 in aqueous solution was investigated kinetically under sonolysis at 20 kHz and under photocatalysis in the presence of titanium dioxide particles, as well as under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis. The influence on the degradation and mineralisation rates of the initial substrate concentration and of the photocatalyst amount was systematically investigated to ascertain the origin of the synergistic effect observed between the two degradation techniques. The evolution of hydrogen peroxide during kinetic runs was also monitored. Small amounts of Fe(III) were found to affect both the adsorption equilibria on the semiconductor and the degradation paths. Ultrasound may modify the rate of photocatalytic degradation by promoting the deaggregation of the photocatalyst, by inducing the desorption of organic substrates and degradation intermediates from the photocatalyst surface and, mainly, by favouring the scission of the photocatalytically and sonolytically produced H(2)O(2), with a consequent increase of oxidising species in the aqueous phase.

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